Single channel receivers can be used to detect and demodulate transmitted signals having a single carrier frequency or central frequency. Examples of such signals are amplitude modulation (AM) signals, single sideband (SSB) and other frequency modulation (FM) signals, and pulse code modulation (PCM) and other pulse modulation (PM) signals. Such signals can be encrypted or otherwise modified to interfere with the ability of a hostile receiver to demodulate, jam or utilize such signals. However, encryption can sometimes be defeated by a sufficiently capable hostile receiver.
To avoid effective intercept or reception of a transmitted signal by a hostile receiver, frequency hopping or spread spectrum transmission has been used. Frequency hopping involves changing the center or carrier frequency of a transmitted signal in a manner already known to a friendly receiver, but intended to be unknown to a hostile receiver. Such changes can be rapid, such as 100 hops per second. Reception of a frequency hopping signal by a single channel receiver is thereby effectively defeated.
Existing frequency hop transmitters, receivers and transceivers often use the 30-88 MHz VHF band, with about 100 hops per second, over from only a nominal 5 MHz band up to over the full 58 MHz VHF bandwidth. Analog FM voice or digital CVSD voice data signals have been used. Other bands and other hop rates have also been used.
A minimum of 90% of the transmitted signal at each frequency hop should be recovered to produce a usable signal. For example, for a transmitter operating at 100 hops per second, its transmitted signal remains at any one frequency for only 1/100 of a second=10 milliseconds per hop. This leaves 10% of 10 milliseconds=1 millisecond for signal detection. A VHF frequency hopping transmitter using the full 58 MHz VHF bandwidth with 25 KHz-wide channels would have 58 MHz/25 KHz=2320 channels. A step-tuned conventional receiver would then have to be able to search all 2320 channels in one millisecond, which is beyond the capability of most conventional receivers. The minimum dwell time per channel of a conventional superheterodyne receiver should be longer than the impulse response of its filter, which is the inverse of its bandwidth, or 1/(25 KHz)=40 microseconds. In the illustrative example, 40 microseconds per channel times 2320 channels equals 92.8 milliseconds, which is much greater than one millisecond; a frequency-hopping transmitter would hop nine times in 92.8 milliseconds.
One approach to detecting frequency hopping transmitted signals has been to perform rapid, wideband spectral analysis to detect the frequency hopper's current active channel or current hop frequency on the basis of frequency, dwell time, direction of arrival, and signal amplitude. A conventional superheterodyne receiver is then tuned to the current frequency of the hopping signal. This approach requires two receivers: one rapid wideband search receiver for hop detection and current frequency identification, and one for demodulation of the actual signal. While such devices have served the purpose, they have not proven entirely satisfactory because complex, heavy, physically large apparatus, requiring substantial amounts of power, is required by this approach. Power requirements may be so large as to prevent or reduce ready portability, physical concealment, electrical concealment, and application to covert use.